1. Technical Field
The invention relates to high temperature superconductors deposited on ferrimagnetic substrates and used to make microwave devices. More particularly, superconducting oxides, such as yttrium barium copper oxide (YBCO), are deposited on ferrimagnetic polycrystalline ceramics, such as yttrium iron garnet (YIG), and used to make practical microwave devices such as phase shifters, circulators, filters, and the like.
2. Background
Ferrite components have played an important role in microwave systems for nearly forty years. Their inherent capability of meeting demanding loss and power-handling requirements make them the technology of choice for many applications. Adjustable phase shifters, for example, are the principal beam-steering component of agile-beam phased-array radars. Ferrite circulators are used in radar systems to isolate transmitters from reflected energy and to direct return signals into receiver channels. Circulators can also be employed as switches in selective filter banks. More recently, magnetically tunable ferrite filters employing YBCO resonator circuits have been demonstrated and are expected to find use in both military and commercial systems.
However, normal metal microstrip, as opposed to waveguide-geometry, ferrite devices have had limited use at microwave frequencies because of high insertion losses resulting from resistance in the metal conductors. U.S. Pat. No. 5,484,765, issued Jan. 16, 1996, to Dionne et al., incorporated herein by reference, discloses Nb superconductor microstrip-geometry ferrite microwave devices. Nb must be operated below 9 K and the use of yttrium barium copper oxide (YBCO) was suggested because it is a high temperature superconductor with a transition temperature of about 90 K that is conveniently warmer than the 77 K temperature of liquid nitrogen. In addition, YBCO has a higher critical current density than other high temperature superconductors and has been much studied.
Results for YBCO-based microwave devices were reported in Dionne et al., "YBCO/Ferrite Low-Loss Microwave Phase Shifter," IEEE Trans. Appl. Supercond., vol. 5, pp. 2083-2086 (1995) and "Ferrite Superconductor Microwave Devices for Advanced Applications" IEEE Trans. Microw. Theory Tech., vol. 44, no. 5, pp. 1361-1368 (1996), both of which are incorporated herein by reference. The phase shifters disclosed in these references provide dramatically improved figures of merit (degrees of phase shift per dB of loss) compared with conventional microwave devices. As described therein, a magnetic toroid, at least one side of which is a flat polycrystalline yttrium iron garnet (YIG) plate is used to produce a magnetic field confined within the toroid. In the YBCO version, a thin film is deposited on a nonmagnetic single crystal lanthanum aluminate substrate with a meanderline pattern and the film side is mechanically pressed against the YIG plate. Microwave fields outside the superconductor interact gyromagnetically with the magnetic field in the YIG to produce substantial phase shift along the length of the meanderline. The toroidal shape keeps the magnetic field inside the toroid so that the superconductor's conduction is not affected. Figures of merit of 1000.degree./dB were obtained compared with 160.degree./dB for non-superconducting metal devices.
However, the magnetic circuit configuration required a mechanical press-fit assembly of a ferrite plate and YBCO deposited on a separate single-crystal substrate. This may be too cumbersome for production quality control and survival in harsh environments. A method of deposition of YBCO films directly on ferrite substrates would be a substantial improvement in realizing these superconductor/ferrite microwave devices. Unfortunately, growing high quality single crystal YBCO films on desirable substrates is not easy. Often, a transitional buffer layer of a more easily grown material must be deposited between the substrate and the YBCO to be either a better match for the YBCO crystal lattice size or prevent diffusion of the substrate material into the YBCO layers or both. A number of buffer materials and substrates have been tried.
One example is disclosed in U.S. Pat. No. 5,262,394, issued Nov. 16, 1993, to Wu et al., incorporated herein by reference. Wu discloses a buffer layer exemplified by CeO.sub.2 deposited by pulsed laser deposition on a variety of single crystal substrates including yttria-stabilized zirconia (YSZ), but not including YIG. This yielded a critical current density, Jc, of about 10.sup.7 A/cm.sup.2 for YBCO films that is close to the best obtained.
Examples using single crystal YIG substrates are disclosed in U.S. Pat. No. 5,635,435, issued Jun. 3, 1997 to Pique, et. al. wherein high quality YBCO thin films were deposited using two buffer layers. A laser ablation technique was used to epitaxially grow films with crystallographic orientations along the normal of [001]YBCO .parallel.[100]BaZrO.sub.3 .parallel.[001]SrZrO.sub.3 .parallel.[001]YIG. As reported in Pique et al., "Microwave Compatible Yba.sub.2 Cu.sub.3 O.sub.7-x films on ferrimagnetic garnet substrates," Appl. Phys. Lett., vol. 67, no. 12, pp 1778-1780 (1995), the YBCO film showed a superconducting transition temperature of 88-89 K with transition widths of about 0.5 K, Jc=2.7.times.10.sup.6 A/cm.sup.2, and surface resistance of 0.5 m.OMEGA. at 77 K nd 10 GHz. YIG is a very desirable substrate material for microwave devices because it has a low dielectric constant and a low loss tangent that reduce power dissipation. However, at present, single-crystal YIG is not available in large enough sizes and quantities for commercial devices and polycrystalline ceramic versions are usually preferred.
Growth on polycrystalline substrates presents an additional challenge because high critical current density and low microwave surface resistance are achieved only in near single-crystal YBCO films that are normally obtained through epitaxial growth on lattice-matched single-crystal substrates. The reason is that any high-angle grain boundaries mismatched in the a-b plane of YBCO films have the properties of Josephson weak links and lower macroscopic transport currents across the grain boundaries. Thus, a crystalline template for the YBCO must be grown on the polycrystalline substrate. Y. Iijima et al. "In-Plane Texturing Control of Y--Ba--Cu--O Thin Films on Polycrystalline Substrates by Ion-Beam Modified Intermediate Buffer Layers," IEEE Trans. Appl. Superconductivity, vol. 3., no. 1, March 1993, and U.S. Pat. No. 5,650,378, issued Jul. 22, 1997 to Iijima et al. (both of which are incorporated herein by reference) disclosed a deposition technique called Ion-Beam-Assisted-Deposition (IBAD). This technique was used to deposit on polycrystalline nickel alloys polycrystalline but biaxially oriented cubic YSZ films. In this orientation, in all the different grains, one axis is normal to the substrate and the other two axes are aligned within a few degrees of each other, i.e., the grain boundaries are at small angles. Using the YSZ as a template, conventional pulsed laser deposition was used to deposit biaxially oriented YBCO films with few high-angle grain boundaries. However, the critical current density of 4.3.times.10.sup.5 A/cm.sup.2 at 77 K was lower than that obtainable with YBCO films on single crystal substrates.
Findikoglu, et al., "Microwave surface resistance of YBa.sub.2 Cu.sub.3 O.sub.7-x films on polycrystalline ceramic substrates with textured buffer layers," Appl. Phys. Lett., vol. 69, pp. 1626-1628 (1996) used the IBAD technique to grow YSZ on polycrystalline alumina. (Alumina is a microwave substrate material that can be highly polished and its thermal expansion coefficient matches that of YBCO, but it is not ferrimagnetic.) Pulsed laser deposition techniques were used to deposit an additional CeO.sub.2 buffer layer followed by YBCO. The best 10 GHz microwave surface resistance was measured as 1.89 m.OMEGA. that, according to the authors, is five times higher than comparable films grown on single crystal YSZ. A strong empirical inverse correlation with the average spread in grain boundary orientations was noted, but a quantitative explanation proved elusive.